Conformational structures of jet-cooled acetaminophen-water clusters: a gas phase spectroscopic and computational study.

Jet-cooled acetaminophen (AAP)-water clusters, AAP-(H2O) (1), were investigated by mass-selected resonant two-photon ionization (R2PI), ultraviolet-ultraviolet hole-burning (UV-UV HB), infrared-dip (IR-dip), and infrared-ultraviolet hole-burning (IR-UV HB) spectroscopy. Each syn-and anti-AAP rotamer has three distinctive binding sites (-OH, 4CO, and 4NH) for a water molecule, thus 6 different AAP-(H2O) (1) conformers are expected to exist in the molecular beam. The origin bands of the AAP(OH)-(H2O) (1) and AAP(CO)-(H2O) (1) conformers (including their syn-and anti-conformers) in the R2PI spectrum are shifted to red and blue compared to those of the AAP monomer, respectively. These frequency shifts upon complexation between a water molecule and a specific binding site of AAP are also predicted by theoretical calculations. The spectral assignments of the origin bands in the R2PI spectra and the IR vibrational bands in the IR-dip spectra of the four lowest-energy conformers of AAP-(H2O) (1), [ syn-and anti-AAP(OH)-(H2O) (1) and syn-and anti-AAP(CO)-(H2O) (1)], are aided by ab initio and time-dependent density functional theory (TDDFT) calculations. Further investigation of the IR-dip spectra has revealed a hydrogen-bonded NH stretching mode, supporting the presence of the syn-AAP(NH)-(H2O) (1) conformer. Moreover, by employing IR-UV HB spectroscopy, we have reconfirmed the existence of the syn-AAP(NH)-(H2O) (1) conformer, which happened to be buried underneath the broad background contributed by the AAP(OH)-(H2O) (1) conformers. These observations have led us to conclude that all of the possible conformers of AAP-(H2O) (1) have been found in this study.